1. Field of the Invention
This invention relates to coverings for sealing and protecting surfaces. More particularly, this invention relates to transferable, scratch-resistant cover layers for sealing and protecting graphic-arts images, particularly, large size, full color images.
2. Description of Related Art
A variety of graphic arts processes are used in the printing, office copy, decorative arts and advertising industries. Such graphic arts processes include conventional processes to produce photographs; litho films; pre-press proofs; printed publications; xerographic, electrographic, and ink-jet images; and the like. In preparing such graphic-arts, image-bearing surfaces, it is frequently desirable to provide a protective covering to prevent limited scratching, marring, or other damage to the image-bearing surface. Such a covering should be optically clear so as not to obscure or distort the image. However, such a covering may lower the gloss of the surface. The covering should also have flexibility, durability, and good adhesion to the image-bearing element.
Graphic-art elements have been provided with a great variety of protective coverings. Fulwiler, U.S. Pat. No. 4,077,830 teaches that photographic elements such as positives or negatives used for contact printing can be protected by application of a thin transparent sheet of resin material having a thin coating of a transparent pressure-sensitive adhesive. To avoid image distortion during printing, both the resin sheet and the adhesive layer must be extremely smooth. The preferred protective resin sheet is a high gloss polyethylene terephthalate film.
Taylor, U.S. Pat. No. 4,921,776 discloses a method of providing a lower gloss protective covering for a pre-press color proof having an image-bearing surface. In the disclosed process, a thin, substantially transparent integral film, which consists essentially of a mixture of at least two slightly incompatible polymers, is laminated to the image-bearing surface. A second "wetting" or adhesive layer may be disposed between the integral film and the image-bearing surface. Also disclosed, are additional layers adjacent to the protective and adhesive layers to facilitate handling and storage. The additional layers are removed during the lamination process. Suitable materials disclosed for these additional coverings are silicone-treated polyester, polyethylene, etc., which release readily from the layer they serve to protect. Silicone-treated polyethylene terephthalate support is specifically exemplified.
Conder, U.S. Pat. No. 4,343,855 discloses preformed abrasion resistant coating layers derived from epoxy-silanes or acryloxy-silanes. The layers are applied to surfaces by means of a composite comprising a release liner, the abrasion resistant layer, and a thermosoftenable adhesive layer. It is disclosed that the release liner may be formed of smooth polymeric film to which the abrasion resistant coating composition is not readily bondable (e.g., polyester, polyolefin, etc.), or silicone treated surfaces. Specifically exemplified is the extrusion bar coated abrasion resistant coating composition onto polyethylene terephthalate film. It is further disclosed that the thermosoftenable adhesive layer must be coated onto or combined with the abrasion resistant layer prior to complete curing of the abrasion resistant layer, since that layer has been found to be so completely non-reactive when fully cured that thermoplastic layers will not readily bond to its surface.
Although advances have been made in the protection of graphic arts images, conventional systems and processes are still limited. In particular, even the thinnest of the currently used laminating films impart a significant curl to the side of the image on which the film is applied. In order for the image to lay flat, an offsetting lamination typically is carried out on the opposite side of the image which essentially doubles the expense for providing the needed protection. In instances when sections of the protected image element are cut out by die cutting processes, current laminating films can cause excessive wear to cutting edges of die cutting equipment leading to increased replacement costs and production down-time. When other means are used to protect images, such as by applying overcoats of varnish or other clear lacquers, image distortion may occur due to the chemicals and solvents employed, and safety and environmental concerns typically are enhanced. Accordingly, there continues to be a need for a simplified transfer composite to provide distortion-free protection to graphic-arts images, and particularly, for use on large format posters, billboards and the like. In addition to abrasion resistance, there is also a need to provide such images with sealed protection from environmental or ambient effects and components which degrade the image.